UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Hum pickup on inter-stage coupling transformer

Chris55000 · 29th May 2020, 10:42 pm

Hi!

It might be worth while seeing if you can cancel it out by means of a 220 ohm preset across the heater supply of the microphone preamplifier, connecting the wiper to the supressor–grid (G3) of your preamplifier valve with a small capacitor of about 10n added from G3 to chassis.

It is possible to cancel out hum with a specialist "Wheatstone Bridge" technique in the output stage, but it involves alterations to the G.B. arrangements and needs careful adjustments!

Chris Williams

29th May 2020, 10:42 pm	#20
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,874	Re: Hum pickup on inter-stage coupling transformer Hi! It might be worth while seeing if you can cancel it out by means of a 220 ohm preset across the heater supply of the microphone preamplifier, connecting the wiper to the supressor–grid (G3) of your preamplifier valve with a small capacitor of about 10n added from G3 to chassis. It is possible to cancel out hum with a specialist "Wheatstone Bridge" technique in the output stage, but it involves alterations to the G.B. arrangements and needs careful adjustments! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!